Advances in the field of solid state electronics have recently permitted spectacular improvements in the performance of consumer entertainment audio and video equipment. As a result, the consumer has appropriately come to expect ever higher levels of performance from mobile audio systems. Similarly, consumer tolerance for less than ideal performance is decreasing.
Although it has proved possible to increase the quality of mobile audio amplifiers, recorded media playback units, and output speakers, it has proved more difficult to overcome the difficulties inherent in mobile reception of RF signals, particularly frequency modulated signals, for various reasons. First, the advances in technology which have lead to the demand for improved reception have similarly resulted in a proliferation of RF signal sources. This includes not only numerous additional television and FM commercial stations, but also vastly increased use of the electromagnetic spectrum both above and below, for example, the FM broadcast range (88-108 MHz). As discussed in detail below, this has resulted in a crowding of the broadcast spectrum, particularly in metropolitan areas.
Second, because of inherent propagation restrictions, commercial television and FM broadcasts are somewhat limited in their ideal reception range. However, consumer intolerance for low quality reception, coupled with the long distance mobility of drivers, has lead to a demand for increased sensitivity in, for example, FM receivers to maximize the usable reception range. This is particularly true in regions where desired stations are few in number and spaced relatively large distances apart, such as in the central and southwest regions of the United States.
In order to provide increased sensitivity and improved reception of distant or weak signals, modern automobile FM receivers, for example, typically include an initial RF amplification stage connected to the automobile antenna to boost the signal prior to detection. In this manner, signals that would otherwise fall below the detection threshold are amplified sufficiently to be detected. Unlike the IF amplifier, however, this first stage must process the entire desired spectrum of FM frequencies, since the amplification occurs prior to mixing. Common base amplifier configurations are typically used to provide gain at the RF frequencies.
In a related fashion, external preamplifiers are known, and are used to similarly amplify the broadcast antenna signal. Switches are often provided so that the user may activate or deactivate such amplification as desired.
Difficulties arise when such systems are operated within the crowded and high energy spectrum present in metropolitan areas. Specifically for example, the front end RF amplifiers in FM receivers are limited in dynamic range. Signals which exceed this range saturate the amplifier and are "clipped." Although such clipping is desirable in the IF stage, which is limited to the single desired signal, it can lead to severe intermodulation distortion when multiple signals are present, as in the broadband amplifier. The resulting intermodulated signals may appear at beat frequencies within the audio range, and may cause "phantom" reception of undesired signals, obliterating weaker desired signals. In general, the overloading results in a loss of the desired signal and a highly objectionable high frequency audio distortion.
Clear reception of FM signals in powerful urban settings is further complicated by the rapid and large variations in overall broadcast signal strength that are encountered. For example, at certain locations the antenna may be subjected to very high signal strengths from many RF transmitters, as well as to reflected signal energy. In contrast, other areas lie largely within electromagnetic shadows, and the overall signal strength presented to the antenna may be quite low. In fact, it has been found that for commercial FM broadcasts, fluctuation of as much as 90 dB may exist between nearby locations. Further, a moving car may be exposed to rapid fluctuations of this magnitude. In fact, changes on the order of 60 db may occur within milliseconds as the car is operated normally in a city.
In order to reduce the intermodulation distortion cause by front end overloading, the broadband RF amplifier may be made less prone to saturation. However, this typically reduces the sensitivity, compromising fringe area reception. Automatic gain control circuits are also employed to adjust the gain of the IF amplifier and/or RF amplifier in response to the signal levels within the IF stage. However, such AGC circuitry must be integral to the specific receiver design, and will not prevent intermodulation distortion resulting from the effect of multiple signals exceeding the limited dynamic range of the front end of the receiver.
Finally, certain commercially available automobile receivers incorporate fixed attenuators which are switch-selectible by the user. Commonly denominated as "local/distant" adjustment, the user selectible attenuator provides a predetermined attenuation of the input RF to minimize overloading. However, because of the very wide variations that are present, it is impossible to optimize the receiver to all conditions in this manner, for a preselected attenuation which is sufficiently low to provide the sensitivity required in relatively quiet areas of a city will be insufficient to prevent intermodulation distortion in the high signal areas.
A further drawback of existing systems is the requirement for user intervention. Present systems require that the user first observe the degraded performance of the receiver, then determine its probable cause and possible solution, and finally take the affirmative steps required to engage the appropriate corrective equipment. The driver is thus distracted from the driving function, and is necessarily subjected to temporarily degraded performance in the receiver equipment. Further, it is not possible to provide manual adjustment responsive to the very rapid fluctuations in signal strength present in urban settings.
In view of the foregoing, it becomes an object of the present invention to provide an interface for insertion between an antenna or other source of broadband RF signals and the associated RF receiver for processing the received RF signal to maintain an output signal that is within the optimum input dynamic range of the receiver regardless of the character of the input signal. A related object is to provide signal processing in an automated fashion that does not require user intervention.
A particular object is to provide such an automated signal processing interface for use in connection with mobile receivers, such as automotive FM radio or television receivers.
In this regard, a further object is to provide automatically variable attenuation of undesirably strong received signals so that the resulting output signal is optimally matched to the input requirement of, for example, a standard automobile FM radio, where said automatically variable attenuation is of sufficient range and resolution to maintain the resulting output signal within the desired input range of the automobile radio for substantially all levels of strong input signals.
Another object is to provide sufficiently rapid response of the variable attenuator to compensate for rapid signal strength fluctuations. It is also an object of the present invention to provide for the desirable attenuation in a fashion that prevents "gaps" or intermittent occasions of intermodulation distortion between various attenuator levels, and without introducing additional sources of potential distortion.
On the other hand, it is still a further object of the present invention to also provide for processing of undesirably weak signals when appropriate to improve the sensitivity of the resulting radio system. A related object is to provide for such amplification in an automated fashion that does not require user intervention, and does not interfere with or compromise the aforedescribed objects.
Another object of the present invention is to provide a system that renders the automobile FM receiver, for example, immune from the effects of extraneous signals outside of the desired FM broadcast band. A related object is to make the aforementioned signal processing interface apparatus substantially immune from extraneous operation as a result of signals outside of, for example, the desired FM band.
Another object is to provide an optimizing system as described that permits the user to defeat all or some of the improvement features in order to observe their effect and benefit by comparison to an unprocessed antenna signal.
An important object of the present invention is to provide for such an improved optimization system in a manner which can be utilized in conjunction with existing automotive or other mobile receiver apparatus, such as standard FM radio receivers or tuners, without modification. A related object is to provide such a system for after-market, add-on installation to existing systems, such that the unit operates independently of the particular radio receiver employed. Yet another object is to provide such an optimizing signal processing interface for FM signals, without degrading the performance of the AM receiver in standard AM/FM mobile radios.
A further object is to provide for such an improved optimization system at a minimum of cost and in a fashion amenable to ease of installation and operation by the untrained user.
These and other advantages, objects, and features of the present invention will become apparent in light of the present specification and accompanying drawings.